D3 Human Evolution

Assessment Statements Outline the method for dating rocks and fossils using radioisotopes, with references to 14 C and 40K. Define half-life Deduce the approximate age of materials based on a simple decay curve for a radioisotope. Describe the major anatomical features that define humans as primates. Outline the trends illustrated by the fossils of Ardepithecus ramidus, Australopithecus including A.afarensis and A. africanus, and Homo including H. habilis, H. erectus, H. neanderthalensis and H. sapiens. State that, at various stages in hominid evolution, several species may have coexisted. Discuss the incompleteness of the fossil record and the resulting uncertainties about human evolution. Discuss the correlation between the change in diet and increase in brain size during hominid evolution. Distinguish between ‘genetic’ and ‘cultural’ evolution. Discuss the relative importance of genetic and cultural evolution in the recent evolution of humans.

Dating Fossils with Radioisotopes Most of our understanding of our human origins has come from looking at the remains of our fossil ancestors. Fossils record the appearance of our long-dead ancestors and their locations tell us where they may have lived, but if we can also estimate the age of the fossils, we can begin to construct our human history.

Elements exist in different forms called isotopes, which have different number of neutrons in their atoms.

Carbon exists as different isotopes carbon -12 (12C) accounts for 98% of carbon atoms, while carbon 14 (14C) is found in tiny quantities. Carbon 14 is known as a radioisotope because it is unstable and slowly decays to stable carbon 12. The rate of decay of any isotope is constant. For carbon 14 it is known that after 5,700 years, half the unstable atoms in a sample of carbon will have decayed, so this time period in known as its half-life.

Graph showing the decay of radioisotope carbon 14. If the percentage of the remaining carbon 14 in a specimen is 50%, then one half-life has passed. If the percentage is 25% then two half-lives have passed. Carbon dating of fossil specimens with these percentages would us that the first fossil is around 5,700 years old and the second is 11,400.

Carbon and potassium are used for dating fossils. Carbon 14 has a half life of 5,700 years, and is useful from dating fossil material 1,000-10,000 years old. Potassium 40 has a half life of 1.3 billion years and is used to date very old fossil material or rock.

All naturally occurring carbon and potassium are mixtures of isotopes of known proportions, so during its lifetime an organism will accumulate both forms in its tissues as it feeds or photosynthesizes. When organisms die no more atoms are accumulated so the naturally occurring ratio will begin to change. As radioisotopes decay, their proportion in the organism will decrease and the proportion of the stable form will increase. To date fossil material, the proportion of different isotopes is measured to assess the number of half-lives that has passed, this is radiocarbon dating.

Humans are Primates Taxonomy is based on shared characteristics. Humans are classified as primates, a group within mammals, which includes lemurs, gibbons, monkeys and great apes. Humans share many physical and genetic characteristics with all these animals. Humans are most closely related to the great apes (orangutan, gorillas and chimps. We share the majority of genes with chimps but the base sequences still differ in many positions. Chimps are the closets related to humans based on sequence comparison.

Shared Characteristics Long limbs and grasping hands with opposable thumbs allowing for object manipulation. Extremely mobile joints at shoulder and hip, the ball and socket joints giving great flexibility of movement. Rotating forelimb, which increases the use of the hand. Retention of collar bone, which stabilizes the shoulder allowing body weight to be supported by the arms when moving through trees. Forward-facing eyes and flattened face, which permits stereoscopic vision

Trends in the Human Fossil Record Hominids are a group of primates that walk on two legs. Modern humans are members of this group alive today but many other species of hominids have existed in the pat, and some of them were alive at the same time. Our knowledge of these species has been gained entirely from the fossils that we’ve found.

Ardipithecus ramidus 4.4-4.3 mya in Ethiopia, East Africa Foramen mangnum is more ventral, indicating a trend towards bipedalism. Brain size unknown Teeth seems to be intermediate between apes and Australopithecines canines are smaller and more blunt than apes but molars are large and ape-like

The point at which the spinal column meets the skull is the foramen magnum. A ventral foramen magnum allows for a more upright body position and bipedal walking.

Australopithecus afarensis 3.9-2.9 mya in East Africa Most well known specimen is Lucy Ape-like face with flat nose and protruding jaw Large, tall lower jaw Brain size: 375-550 cm3 Large molar teeth similar to A. ramidus

Australopithecus africanus 3.3-2.5 mya in Southern Africa Slightly flatter face but still with large, tall lower jaw Brain size:420-500cm3 Smaller canines but molars still large

Homo habilis Used simple tools 2.5-1.9 mya in eastern and southern Africa Face more flattened than Australopithecus, smaller lower jaw bone Brain size: 500-800cm3 Smaller teeth, hips form distinct pelvic bowl

Homo erectus 1.8-0.3 mya in Africa, Indonesia, Asia, Europe Found in Europe and Africa so must have migrated. First hominids to use fire Face further flattened, skull more rounded with large brow ridges Smaller lower jaw Brain size: 850-1100cm3

Homo neanderthalensis 150,000-30,000 years ago in Europe and western Asia Face furhter flattened but still large brow ridges Rounded skull but lower forehead Brain size:1200-1625cm3 Large teeth and jaw muscles, limbs short relative to torso May have interbred with H. sapiens in Europe

Homo sapiens 130,000 years ago Flat face with no brow ridges Reinforced lower jaw producing a chin Rounded skull with high forehead Brain size: 1200-1500cm3 Smaller molars, skeleton is less robust than other ancestors

In at least 2 cases, different hominid species lived in the same place at the same time- A. africanus and H. habilis , and H. neanderthalensis and H. sapiens co-existed in Europe. There is a lot of debate about the degree to which the two species intermingled, but the definitely inhabited the same regions for thousands of years, At the same time, their populations were small and spread out over 3 continents, so they may not have ever met.

Australopithecus sediba skull bears both human and ape traits. Found in 2011, 2 million years old. Both human and ape characteristics. Missing link? Our understanding of human evolution is based on the fossil record, but fossilization is a rare event and discovering a fossil is also rare. The fossil record for humans is very patchy Over the years new fossils have been discovered, dating techniques are more accurate, and genetic analysis has been applied. The branching of the human evolution tree has been modified many times. Some evolutionary paths contain 18 or 19 examples, but some may be variations of the same species.

Diet and Brain Size in Hominid Evolution All the hominid fossils found fit into a sequence showing an increasing degree of adaptation to bipedalism, and increasing brain size relative to body size.

Large brains require a lot of energy to enable them to function and there is evidence to correlate the increase in brain size with changes in diets to our ancestors. H.Sapiens uses 20% of the total energy consumed by the body.

Human ape ancestors lived in northeast Africa in the Great Rift Valley. Until about 5 million years ago this was covered in dense forest, but movements of the Earth created a wide valley, and volcanic activity coated the plains with thick ash, which prevented growth of trees. Vegetarian tree-dwelling apes, which ate soft leaves and fruit, had to adapt or become extinct. Australopithecines adapted by developing stronger jaws and teeth to deal with tougher vegetation such as stems, roots, but as the savannah expanded, the variety of plants decreased and grazing animals started to play a big role in the diet.

Early hominids needed new strategies to access this rich source of meat, which provided an increased supply of protein, fat and energy. Natural selection favored individuals with large brains who could develop new strategies for hunting and work in groups to kill large animals. Meat provided the nutrients to build a larger, thinking brain as well as the energy to fuel its activity

The increase in meat in the diets shows a positive correlation with increased brain size and also with the development of more sophisticated tools for hunting.

Genetic and Cultural Evolution Genetic evolution of humans is the change, over millions of years, in the genome population. Genetic evolution occurs as a result of mutations and these may be inherited. Genetic evolution includes the change to bipedalism, changes in teeth and increase brain size. All these features are passed on from parents to their offspring.

Cultural evolution, on the other hand, does not involve genetic changes and can take place over a very short periods of time, even within a generation. Cultural evolution involves changes in behavior, social organization, ideas, communication, teaching and learning. These are acquired characteristics that do not have a genetic basis, though the ability to acquire them may be genetically determined. Cultural evolution does not result in speciation.

The rise of agriculture brought about an enormous alteration in social organization for humans. Instead of living in small mobile hunter-gatherer groups, humans began to settle in large fixed communities, and the rise of different religious faiths further affected new social organizations. The invention of the printing press had a huge impact on communication. TV, the telephone, and internet.